(1) Field of the Invention
The present invention relates to a semiconductor device for forming a power amplifier, and more particularly to a semiconductor device for forming a multi-stage ultra high frequency power amplifier.
(2) Description of the Related Art
A conventional multi-stage power amplifier of the kind to which the present invention relates is described with reference to and by example of a two stage power amplifier. FIG. 1A is an outline of the constitution of a conventional power amplifier and FIG. 1B is an equivalent circuit diagram thereof. The power amplifier shown in FIGS. 1A and 1B is provided with an input matching circuit 7, a driver transistor 300 constituted from a MES (Metal-semiconductor) type field effect transistor, an inter-stage matching circuit 8, a final or output stage transistor 400 constituted from a plurality of MES type field effect transistors, and an output matching circuit 9. The driver and final stage transistors 300, 400 are discrete components, and are mounted together with the respective matching circuits 7, 8 and 9 on a wiring circuit board.
This type of multi-stage power amplifier is used in great quantities in the power module etc. of cellular handheld telephones and, recently, components for such uses have been subjected to strong demands for miniaturizing and cost reduction. But, because conventional multi-stage power amplifiers use discrete power transistors at each stage, the size of the whole power amplifier, including the matching circuits, on the wiring circuit board becomes inevitably large. Also, in the conventional examples, a large number of components, a wiring circuit board with a large surface area, a large number of assembly and processing stages, all make cost reduction difficult. Also, in the conventional examples, particularly when miniaturizing, oscillation becomes more likely because of the difficulty in obtaining sufficient isolation between the previous and subsequent stages.